A typical liquid crystal display comprises a liquid crystal element or cell, a polarizing sheet and an optical compensator (phase retarder) provided between the liquid crystal cell and the polarizing sheet.
Current rapid expansion in the liquid crystal display (LCD) applications in various areas of information display is largely due to improvements of display qualities. Contrast, color reproduction, and stable gray scale intensities are important quality attributes for electronic displays, which employ liquid crystal technology. The primary factor limiting the contrast of a liquid crystal display is the propensity for light to “leak” through liquid crystal elements or cells, which are in the dark or “black” pixel state. Furthermore, the leakage and hence contrast of a liquid crystal display are also dependent on the angle from which the display screen is viewed. Typically the optimum contrast is observed only within a narrow viewing angle centered about the normal incidence to the display and falls off rapidly as the viewing angle is increased. In color displays, the leakage problem not only degrades the contrast but also causes color or hue shifts with an associated degradation of color reproduction. In addition to black-state light leakage, the narrow viewing angle problem in typical twisted nematic liquid crystal displays is exacerbated by a shift in the brightness-voltage curve as a function of viewing angle because of the optical anisotropy of the liquid crystal material.
Thus, one of the major factors measuring the quality of such displays is the viewing angle characteristic, which describes a change in contrast ratio from different viewing angles. It is desirable to be able to see the same image from a wide variation in viewing angles and this ability has been a shortcoming with liquid crystal display devices. One way to improve the viewing angle characteristic is to insert a compensator (also referred as compensation film, retardation film, or retarder) with proper optical properties between the polarizer and liquid crystal cell, such as disclosed in U.S. Pat. No. 5,583,679 (Ito et al.), U.S. Pat. No. 5,853,801 (Suga et al.), U.S. Pat. No. 5,619,352 (Koch et al.), U.S. Pat. No. 5,978,055 (Van De Witte et al.), and U.S. Pat. No. 6,160,597 (Schadt et al.). A compensation film according to U.S. Pat. No. 5,583,679 (Ito et al.) and U.S. Pat. No. 5,853,801 (Suga et al.), based on discotic liquid crystals which have negative birefringence, is widely used. It offers improved contrast over wider viewing angles. However, it suffers larger color shift for gray level images, compared to a compensator made of liquid crystalline materials with positive birefringence, according to Satoh et al. (“Comparison of nematic hybrid and discotic hybrid films as viewing angle compensator for NW-TN-LCDs”, SID 2000 Digest, pp. 347–349, (2000)). To achieve comparable performance in the contrast ratio while reducing color shift, one alternative is to use a pair of crossed liquid crystal polymer (LCP) films on each side of liquid crystal cell, as discussed by Chen et al. (“Wide Viewing Angle Photoaligned Plastic Films”, SID 99 Digest, pp.98–101 (1999)). This paper states that “since the second LPP/LCP retarder film is coated directly on top of the first LCP retarder film, the total thickness of the final wide-view retarder stack is only a few microns thin”. Although they provide very compact optical component, one of the challenges of this method is to make two LCP layers crossed, particularly in a continuous roll to roll manufacturing process.
In a liquid crystal cell comprising a pair of substrates, a rod-like liquid crystal compound and an electrode layer, the rod-like liquid crystal compound is provided between the substrates. The electrode layer has a function of applying a voltage to the rod-like liquid crystal compound. Each of the substrates has an orientation layer (both-sided orientation layers), which has a function of aligning the rod-like liquid crystal compound. The orientation layer of the liquid crystal cell is usually prepared by forming a polymer (e.g., polyimide, polyvinyl alcohol) membrane on the substrate, and rubbing the membrane with a cloth uniformly.
The alignment of the liquid crystal molecules using a linearly photopolymerizable polymer (LPP) is an alternate method that can improve yield and cost of LCDs. An LPP layer is applied to the surfaces of the LCD substrates and optically aligned, thereby generating the required alignment and bias tilt angle for the liquid crystal molecules in the display. This process replaces the mechanical brushing of the polyimide layer described above that is used in the industry today, and offers a number of distinct advantages. Alignment of the liquid crystals can be in more than one direction within the display. Hence, single or multi-domain pixel structures with sub-micron resolution can be generated, resulting, for example, in novel displays with in-built temperature independent viewing angle compensation. Optical alignment is a non-mechanical, non-contact process, which does not generate dust particles or electrostatic charge, which can damage the TFT's and reduce the yield. Furthermore, the process can be integrated into the manufacturing line and offers the possibility to reduce the overall manufacturing cost. The LPP materials are easy to apply, using conventional coating techniques such as printing or spin-coating. Application can also be carried out on a continuous, roll-to-roll web onto flexible polymer substrates, for use in the manufacture of plastic LCD's. By applying a thin film of an LCP material on top of the LPP layer, and by combining various LPP/LCP layers, as discussed earlier, a wide range of new optically anisotropic solid-state thin-film devices can be created. By varying the composition of the LCP layers, the characteristics of the resultant film (e.g. anisotropy, dispersion, transmission) can be adjusted to suit the end use. Specific design of the formulation of the LCP mixture can also generate the required operating temperature for the manufacture of the films. The resultant effects can be applied to a wide variety of optical displays and devices, giving rise to performance improvements and the generation of new devices.
U.S. Pat. No. 5,583,679, U.S. Pat. No. 6,061,113, and U.S. Pat. No. 6,081,312 describe the use of subbing or undercoat layers to improve the adhesion of an alignment layer and an optically anisotropic layer comprising a discotic liquid crystal material to the support.
It is a problem to be solved to provide an optical compensator that widens the viewing angle characteristics of liquid crystal displays, in particular Twisted Nematic (TN), Super Twisted Nematic (STN), Optically Compensated Bend (OCB), In Plane Switching (IPS), or Vertically Aligned (VA) liquid crystal displays, and is readily manufactured, does not cause unwanted curl of the support and improves the ability of the LPP to align. These various liquid crystal display technologies have been reviewed in U.S. Pat. No. 5,619,352 (Koch et al.), U.S. Pat. No. 5,410,422 (Bos), and U.S. Pat. No. 4,701,028 (Clerc et al.).
The ability to provide the orientation layer at the point of manufacture of the plastic support is a highly desired feature. However, in order to accomplish this feature, the orientation layer should be impermeable to the components of the support, such as plasticizers, UV stabilizers, low molecular weight polymers derived from the support polymer and other additives. This becomes particularly challenging when the LPP layer is coated from an essentially all organic solvent. Such solvents are typically used in high speed film support manufacturing. Typically the LPP layer is extremely thin (under 3 microns) and at the same time functions as the orientation layer for the subsequent LCP layer. Thus, quality of the layer is especially critical and must not adversely affect the optical alignment of the layer since the orientation of the LCP is dependent on efficient alignment of the LCP layer. Contaminants can adversely affect the alignment process.
U.S. Pat. Nos. 6,061,113 and 6,081,312 teach compensator plates for liquid crystal cells but do not provide the desired protection from the migration of support components in the case of polymeric supports. U.S. Pat. No. 5,583,679 also teaches the use of a hardened gelatin layer as a second subbing layer to promote adhesion of the optical alignment layers to the support but no mention is made of the migration of contaminants from the support to the orientation layer.
Therefore, there exists a need for an optical compensator for a liquid crystal display that widens the viewing angle characteristics of liquid crystal displays without significantly degrading the optical properties of the display.